PART II 



THE BLOOD AND LYMPH 

 IMMUNITY 



SECTION IV 

 THE BLOOD 



CHAPTER XIV 

 GENERAL CHARACTERISTICS OF THE BLOOD 



General Consideration. The general body fluid of the lowest 

 organisms possesses the simplest possible composition. It is widely 

 distributed through the intercellular spaces and is separated from the 

 surrounding medium by a very permeable membrane. Being thus 

 fully exposed to osmotic influences from without, different nutritive 

 substances are constantly forced into the organism, while its waste 

 products are made to pass into the medium. These osmotic condi- 

 tions, however, are adjusted in such a way that the general fluid of 

 the body is quite unable to acquire a concentration much above that 

 of the surrounding medium. 



Separate circulatory channels are not present in the lower forms. 

 Instead, the alimentary canal is called upon to perform a double func- 

 tion, namely, that of serving as a receptacle in which the nutritive 

 substances undergo mechanical and chemical reductions, and that of 

 distributing the assimilated material to the different parts of the body. 

 A much higher stage of development is attained in those animals in 

 which the alimentary tract assumes a variegated shape and in which its 

 different recesses eventually become disconnected from the main chan- 

 nel to form the beginnings of the circulatory system. In this manner a 

 number of internal reservoirs are developed, from the contents of which 

 the tissue-cells derive their nourishment directly. But, while the 

 body-fluid is thus more thoroughly protected against outside influences, 

 its isolation is not complete, because it continues to be exposed, on the 

 one hand, to the osmotic power of the contents of the alimentary 

 canal and, on the other, to the conditions prevailing in the cells of the 

 tissues. 



With increasing cellular differentiation, this fluid of the celom 

 gradually assumes the characteristics of real blood. Moreover, as the 

 interior spaces become more variegated, certain elementary forces are 



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